In an attempt to elucidate the role of prolactin (PRL) in normal and neoplastic prostate and its relationship to androgen, this research is using light microscope immunohistochemistry (IHC) to visualize PRL binding sites (PBS) in prostate tissue. Golgi-localized PBS were observed in epithelial cells of male rat ventral prostate while in lateral and dorsal prostate intracellular PBS were more diffuse. The lateral prostate which was aparently the most active in binding PRL was unique in that it had pronounced intraluminal PBS. Intense PBS in apica epithelial blebs was characteristic of dorsal prostate. In ventral prostate a significant diminution in PBS occurred 2 to 4 days post-castration which corresponds in time to the major regressive structural change in ventral prostate. Histologic regression and loss of PBS were much slower in dorsal and lateral prostate but could be accelerated by hypophysectomy. As a complement to the above ablation experiments, we are currently studying the effects of androgen replacement in varying doses on prostate structure and PRL injection or by pituitary transplantation to the kidney. As an outgrowth of our prostate experiments we discovered serendipitously with IHC the existence of specific human placental lactogen (HPL) immunoreactivity in growth hormone cells of rat pars distalis. Since HPL is inherently lactogenic and has prostatrophic effects, HPL in rat pituitary may have relevace in terms of prostatic function. In future experiments, we intend to examine the prolactin-androgen interplay in R3327 rat prostatic cancers.